Chimie Douce (Soft Chemistry)

Approach : Chimie Douce reactions are carried out under moderate conditions (typically T < 500° C). Chimie Douce reactions are topotactic, meaning that structural elements of the reactants are preserved in the product, but the composition changes.

Advantages : Chimie Douce Methods are very useful for the following applications:

• Modifying the electronic structure of solids (doping)
• Design of new metastable compounds (structural motif can be selected by choice of precursor, may have unusual properties)
• Preparing reactive and/or high surface area materials used in heterogeneous catalysis, batteries and sensors

Disadvantages : First of all, one must find the appropriate precursor in order to carry out chemie douce. Secondly, metastable products are often unstable in applications where high temperatures are used or single xtals are needed

Intercalation

Involves inserting ions into an existing structure, this leads to a reduction (cations inserted) or an oxidation (anions inserted) of the host.

Typically carried out on layered materials (strong covalent bonding within layers, weak van der Waals type bonding between layers, i.e. graphite, clays, dicalchogenides, etc.).

Performed via electrochemistry or via chemical reagents as in the n-butyl Li technique.

Examples :

TiS₂ + nBu-Li ® LiTiS₂

b-ZrNCl + Naph-Li ® b-Li_xZrNCl

De-intercalation

The reverse of intercalation, also performed using either electrochemical methods or with reactive chemical species

Examples :

NiMo₃S₄ ® Mo₃S₄ (Wash with HNO₃)

In₂Mo₆S₆ + 6HCl (g) ® Mo₆S₆ + 2InCl₃ (g) + 3H₂ (g)

This approach can often lead to new phases (polymorphs) of previously known compounds

CuTi₂S₄ ® cubic TiS₂

KCrSe₂ ® layered CrSe₂

Li₂FeS₂ ® FeS₂

Dehydration

By removing water and/or hydroxide groups from a compound, you can often perform redox chemistry and maintain a structural framework not accessible using conventional synthesis approaches

Examples :

Ti₄O₇(OH)_2*nH₂O ® TiO₂ (B) (500° C)

2KTi₄O₈(OH)_*nH₂O ® K₂Ti₈O₁₇ (500° C)

Ion Exchange

Exchange charge compensating, ionically bonded cations (easiest for monovalent cations)

Examples :

LiNbWO₆ + H₃O⁺ ® HNbWO₆ + Li⁺

Cubic-KSbO₃ + Na⁺ ® Cubic-NaSbO₃ + K⁺